To date, the sunlight has been used after transduced into thermic energy or electric energy. In case of using the sunlight as thermic energy, the thermic energy thereof has been used after being absorbed by some apparatus for absorbing thermic energy as a solar hot-water generator for example. In case of using the sunlight as electric energy, the sunlight has been used after transduced into electric energy by solar cells.
By the way, the solar energy absorbed by the apparatus for absorbing thermic energy or solar cells consists of various rays different in wavelength which irradiate the surface of the earth. And the solar energy which can reach the earth's surface is only about 1 kilowatt per square meter. The energy is very thinner than the energy derived from heat of combustion. So, in case of using the solar energy, it has to be condensed.
For example, in case the apparatus for absorbing thermic energy is a solar hot-water generator, the sunlight should be condensed in order that water might be heated as hot as possible. In case of generating electricity from sunlight, nowadays it costs 35 years to generate electricity for making the solar cells. So generating electricity from sunlight with solar cells isn't of practical use. Generating electricity from sunlight as such can increase the electric power surely on condition that the sunlight may be condensed, because the efficiency of generating electricity with the solar cells doesn't depend on the density of the sunlight.
In brief, if I can increase the quantity of light which the solar cells will receive with a certain way much cheaper than with a way of increasing other solar cells, the cost of equipment per unit electric energy will be able to go down. In this way, in case of using the thin solar energy, it is better to use the solar energy after having condensed. A convex lens, a Fresnel lens and a concave mirror and so on are thought as means for condensing light.
However, in case of using a convex lens as the means for condensing sunlight as described before, the central part of lens has become thicker in accordance with condensing the light more widely. And the cost of equipment has been more expensive. So the cost of equipment per unit electric energy has not been able to go down, and the transmission factor of the sunlight has decreased.
In addition to this, the direction to which the sunlight is emitted has changed every moment during the day, and the directions of the lights transmitted through the convex lens have been various as a matter of course. Therefore, in case of fixing the convex lens to a proper position, sometimes the sunlight has been condensed out of the apparatus for absorbing thermic energy or solar cells in accordance with the direction of the sunlight is emitted. Then the sunlight has not been able to be used effectively.
In case of using the concave mirror as means for condensing the sunlight, the system has not been cheap enough for being made. And especially in case the position and direction of the concave mirror are fixed, the sunlight has not been able to be condensed effectively in accordance with the direction of the sunlight.
In case of using the Fresnel lens as means for condensing the sunlight, the system has been cheap enough for being made. And the transmission factor of the sunlight never decreases in accordance with the scale of the system. At these points the Fresnel lens is better than the convex lens or the concave mirror. But another point the Fresnel lens is not always better than the convex lens or the concave mirror. It is that the Fresnel lens is not able to condense the sunlight in accordance with the direction of the sunlight is emitted as in case of the convex lens or the concave mirror.
In brief, in case of condensing sunlight with these existing means for condensing light effectively, the inclination angle of the means for condensing light has to be made follow the direction of the sunlight is emitted every moment. So the system for condensing light has been more expensive and it has cost great deal of time and labor to maintain the system.
In addition to this, when the sunlight scatters and the direction of the sunlight is irregular as on a cloudy day, the sunlight has never been condensed by existing means for condensing light any more.